Twenty-one childhood acute lymphoblastic leukemias were characterized immunologically by a series of classic markers--EAET rosette and surface and cytoplasm immunoglobulins--, a panel of 14 monoclonal antibodies --OKla 1, OKT3, 4, 6, 8, 9, 10, 11, cALLA, Leu-1, FMC-7, FMC-8 and 3A-1--and by assessment of terminal deoxynucleotidyl transferase enzyme. Patients were distributed in five immunological subgroups: 1) Unclassifiable/null ALL (la +, TdT+, cALLA-), one patient, 2) common ALL (la+, TdT+, cALLA+) 15 patients, 3) pre-B ALL (la+, TdT+, cALLA+, clg+) one patient, 4) B-LL (la+, TdT-, cALLA-, slg+) 3 patients, 5) T-ALL (TdT+, EAET+, la-, cALLA-), one patient, and the clinical, biological, morphological and cytochemical peculiarities characterizing each immunological variant are discussed.